When used in motor vehicles, in particular, an ever-present supply potential, combined with temperature and moisture influences as well as ion-supplying substances such as de-icing salt, wiper fluid surfactants or plastic additives, may result in corrosion of contact materials, degradation of plastics or migration on printed circuit boards, producing conductive paths having unwanted cross or fault currents. Over time, these paths become increasingly less resistant, so that the fault currents rise and any surrounding plastic parts may overheat or ignite. However, since the fault currents are often below the response threshold of automotive. fuses, it is not possible to detect and avoid them, in particular in the case of continuous plus wires, by activating the automotive fuses or by interrupting the connection to the battery via an ignition key.
An electric consumer, which is connectable to an a.c. voltage via a connector and two supply lines and has a sensing conductor and a detector circuit for detecting a fault current due to low-resistance shorting of the electric consumer produced by water or another conductive liquid, is known from U.S. Pat. No. 5,159,517. The connector also accommodates an interrupter circuit which interrupts the two supply lines if an overcurrent is detected. The detector circuit may be situated in either a housing of the electric consumer or in the connector, the sensing conductor in the latter case being run from the electric consumer to the connector, together with the two supply lines, as a third conductor.
Another known method for detecting and avoiding fault currents in power networks involves the use of fault current circuit breakers which cut off the entire power supply in the event of a fault. However, it is not possible to use a procedure of this type in motor vehicles, since this may result in dangerous, undefined operating states. Furthermore, there is no need for this, since the vehicle voltage is usually below the level of the safety extra-low voltage.